The measurement of the position of a moving target is frequently performed by an airborne radar. The elapsed time between the transmission of a burst of energy and its reception after reflection from a target is a measure of the distance. However, the transmission of the energy by the radar reveals its presence to a potential enemy who may be "listening" for these transmissions. It is thus advantageous to develop a technique for determining the position of a moving target passively; that is, by receiving energy without first transmitting it.
Previous methods of passive target location typically involve vectoring and referencing a sensor, or by referencing a plurality of omnidirectional sensors and comparing their respective signals. The target to be detected may be either a stationary emitter or a moving vehicle.
Many passive detection systems have been proposed in the prior art, as for example that shown in U.S. Pat. No. 2,940,076, issued June 7, 1960, to T. B. Bisset et al, and U.S. Pat. No. 3,304,409, issued Feb. 14, 1967, to C. Snowdon and R. A. Bond. One such technique is a variation of one that has been used by sailors from antiquity to avoid collision. Basically, the observing, or test craft detects energy waves and modifies its own course until the measured bearing to the target does not change with time. At this point the test craft undergoes a further maneuver and the range may then be calculated. This technique may be ineffective against targets moving at a velocity greater than the velocity of the observer since a zero bearing rate is not always possible. Further limitations develop when the target is a non-continuous emitter and when the system is required to locate more than one target at a time.
Other prior art systems, such as that disclosed in U.S. Pat. No. 3,982,246, issued Sept. 21, 1976, to B. H. Luber, require "continuous" angular measurements which in practice may not be obtainable when the test craft is undergoing dynamic maneuvers or when the target is a non-continuous emitter. Furthermore, such a system requires integrators, differentiators, gyros, accelerometers, and resolvers, all of which introduce their own errors into the system. For use against more than one target, the system would necessitate duplication of much of the bulky analog equipment. The present invention avoids these restrictions.
Additional prior art techniques involve the use of a variety of optical, electromagnetic, or acoustic sensors, as well as the use of cooperating ground-based receiving and transmitting stations. Many such techniques also require a plurality of cooperating sensor units and additional cross-correlator equipment.
The present invention utilizes a novel technique for determining the position of a target for use when the target is radiating energy signals of its own, or reflecting signals which have been generated elsewhere. This technique is effective against either continuous or pulsed energy emitting systems, and it requires only one, rather than cooperating, test platform. Four bearings, taken at discrete and arbitrary times, together with the known course traveled by the test aircraft between the measurements are necessary and sufficient for calculating the target's distance from the test platform. The course traveled by the test aircraft is easily determined from a navigation system such as is typically included in any military aircraft.
One mission for which this technique is intended is airborne early warning (AEW). In this mission, the target may be hundreds of miles from the AEW aircraft. This scenario offers certain advantages and makes certain necessary assumptions realistic. One such assumption is that the target aircraft is not maneuvering. Aircraft generally avoid maneuvers during most of their flight to conserve fuel, and undergo maneuvers only near their targets.
A second assumption inherent in the explanation which follows is that the AEW and target aircrafts are in the same horizontal plane. This assumption is valid because the large distances separating the horizontally moving AEW and target aircrafts result in small vertical angles between them for reasonable altitude separations.